1. Field of the Invention
This invention relates to a catalyst for purification of exhaust gases exhausted from internal combustion engines of automobiles and the like capable of purifying carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) contained in the exhaust gases.
2. Related Art
As a conventional catalyst for purification of automobile exhaust gases, there has generally been known a catalyst comprising a support substrate, a catalyst carrier layer formed on the support substrate and catalyst ingredients loaded on the catalyst carrier layer. And, so far, there have been demonstrated various kinds of catalyst for purification of exhaust gases aiming at improving the purification property.
For example, some catalysts using cerium (Ce) have been disclosed in Japanese Examined Patent Publication (KOKOKU) Nos. 41775/1984 and 20307/1983, and Japanese Unexamined Patent Publication (KOKAI) No. 90695/1984. In these catalysts for purification of exhaust gases using cerium, the cerium is present in the oxide form. The cerium improves the purification property by discharging or storing oxygen (oxygen storing capability) according to the reaction shown in the formula (1) and by controlling the oxidation reaction of CO and HC and the deoxidation reaction of NOx. EQU CeO.sub.2 .revreaction.CeO.sub.2-X +X/2O.sub.2 ( 1)
It has been known that the reaction shown in the formula (1) occurs on the particle surface of cerium oxide. However, in the above mentioned conventional catalysts for purification of exhaust gases, the total surface area of the cerium oxide particles was reduced by the growth of the cerium oxide particles on using the conventional catalysts at elevated temperatures of 800.degree. C. or more. Accordingly, there arises a disadvantage that the purification property has been degraded due to the worsening oxygen storing capability.
Also, some catalysts using other rare earth element and transition metals simultaneously with cerium for aiming at stabilization of activated alumina, have been disclosed in Japanese Examined Patent Publication (KOKOKU) No. 7537/1985, Japanese Unexamined Patent Publication (KOKAI) No. 18180/1973, U.S. Pat. No. 3,951,860 and U.S. Pat. No. 4,170,573. For example, a catalyst disclosed in Japanese Examined Patent Publication (KOKOKU) No. 7537/1985 discloses to use cerium and lanthanum (La) simultaneously to form composite oxide shown in the formula (2). EQU Ce.sub.1-X La.sub.X O.sub.2-X/2 ( 2) EQU (0.3.ltoreq.x.ltoreq.0.5)
In this catalyst for purification of exhaust gases, lattice defects with oxygen vacancies are formed in the fluorite structure of the composite oxide, so that the oxygen storing capability with durability can be achieved. Although the growth of cerium oxide particles is not so fast as that of the catalyst using cerium only, the growth of cerium oxide particles is increased and the purification property is worsened even in this catalyst for purification of exhaust gases.